1. Field of the Invention
The present invention relates to a scanning apparatus having a cascade scanning optical system. The scanning apparatus is provided with a plurality of laser scanning optical systems which are arranged along the main scanning direction and controlled to operate in synchronization with each other so as to realize a wide scanning line.
2. Description of the Related Art
A cascade scanning optical system having a plurality of laser scanning optical systems arranged along the main scanning direction to realize a wide scanning line is known. Such a type of scanning optical system is disclosed in Japanese Laid-Open Patent Publication No. 60-35712 (published on Feb. 23, 1985) or Japanese Laid-Open Patent Publication No. 61-11720 (published on Jan. 20, 1986). Each publication discloses a cascade scanning optical system having a pair of laser scanning optical systems each having a laser beam emitter, a polygon mirror serving as a deflecting device, an f.theta. lens, etc. The pair of laser scanning optical systems are synchronously driven to emit respective scanning laser beams to a photoconductive surface (scanning surface) of a photoconductive drum on a common line thereon extending in parallel to the axial direction of the photoconductive drum, i.e., extending in the main scanning direction. The pair of scanning laser beams respectively scan two adjacent ranges of the common line on the photoconductive surface so as to scan the photoconductive surface of the photoconductive drum in the main scanning direction in a wide range.
There is a fundamental problem to be overcome in such a type of cascade scanning optical system. Namely, how can a scanning line, made on the photoconductive drum by the scanning laser beam emitted from one laser scanning optical system of the cascade scanning optical system, be accurately aligned with another scanning line, made on the photoconductive drum by the scanning laser beam emitted from another laser scanning optical system of the cascade scanning optical system, so that the scanning lines are not apart from each other nor overlap each other in the main scanning direction, i.e., so as to form a straight and continuous scanning line using a combination of the separate scanning lines.
In the case where each laser scanning optical system of the cascade scanning optical system is designed as a non-telecentric system through which the incident angle of a scanning laser beam relative to the photoconductive surface of the photoconductive drum varies in accordance with a variation in the position of a scanning spot of the scanning laser beam on the photoconductive surface in the main scanning direction, a scanning line made through one laser scanning optical system will not be precisely combined with another scanning line made through another laser scanning optical system on the photoconductive surface of the photoconductive drum, i.e., those scanning lines will be apart from each other or overlap each other by a certain amount in the main scanning direction if the photoconductive surface deviates from its original position even by a slight amount.
In the case where each laser scanning optical system of the cascade scanning optical system is designed as a telecentric system through which the scanning laser beam of each laser scanning optical system is always incident on the photoconductive surface in a direction perpendicular to an axial direction of the photoconductive drum, specifically in a direction orthogonal to generatrices of the photoconductive drum, the aforementioned problem in the case of the non-telecentric system will not occur, but it is necessary for each laser scanning optical system to be arranged at different positions in the sub-scanning direction to differentiate the angles of respective laser beams relative to the photoconductive surface, in order to prevent the respective laser beams from interfering with each other. However, with such an arrangement, if the photoconductive surface deviates from its original position even by a slight amount, a scanning line made through one laser scanning optical system and another scanning line made through another laser scanning optical system on the photoconductive drum will deviate from each other in the sub-scanning direction, and accordingly the former and latter scanning lines will not be combined with each other in the main scanning direction, so that a wide scanning line made by a combination of those scanning lines which extend in the main scanning direction across the photoconductive surface of the drum cannot be formed.
The cascade scanning optical system is generally provided, for each laser scanning optical system, with a laser beam detector fixed at a position to detect the scanning laser beam before the scanning laser beam starts generating a scanning line, so that the scanning-commencement position of writing of each scanning line on the photoconductive surface of the drum can be detected using the laser beam detector. Each scanning line starts to be generated upon a predetermined time interval having been elapsed from the moment at which the laser beam detector detects a scanning laser beam.
However, in the case where one or more laser beam detectors deviates from its original position due to a deformation of the casing to which the laser beam detector is secured, caused by, for instance, a variation in temperature, humidity, etc., the scanning-commencement position of writing of each scanning line on the photoconductive surface also deviates from its original position even if each scanning line starts to be generated upon a predetermined time interval having been elapsed from the moment at which the laser beam detector detects a scanning laser beam. Japanese Laid-Open Patent Publication No. 60-35712 (published on Feb. 23, 1985) discloses a way to overcome such a problem by an arrangement in which only one laser beam detector is provided. Such an arrangement is effective in the case where the laser beam detector deviates from its original position in a direction parallel to the main scanning direction. However, the arrangement is ineffective in the case where the laser beam detector deviates from its original position in a direction non-parallel to the main scanning direction.